1. Field of the Invention
The present invention relates to a focusing screen for use in the finder system of a camera such as a single-lens reflex camera. The present invention also relates to a method of forming a microstructure array suitable for use in the production of such a focusing screen.
2. Description of the Prior Art
With a view to improving the ability of a finder system to detect a lack of sharpness during defocusing, by improving its diffusion characteristics while maintaining its brightness, various focusing screens have been proposed that have a fine regular pattern of high and low spots formed on a matted surface.
FIG. 34 shows an example of such conventional focusing screens. In this focusing screen, the surface of an optical material in plate form is provided with a pattern of regular hexagons that are in close contact with one another and each of which have a conical projection formed therein with the apex being positioned at the center of each hexagon. The distance between each apex is 20 .mu.m and the angle of inclination with respect to the surface of the optical material is set at 10 degrees.
The characteristics of this focusing screen are as shown in FIG. 35 which is a spectrum diagram. The spectrum diagram shows by two-dimensional angular coordinates the diffusion of light that emerges from the focusing screen when a single beam of light falls on the screen in a direction normal to it. The intensity of spectrum is indicated by the size of a circle in the diagram.
The three quadrants in the diagram correspond to the wavelengths of R (637 nm), G (555 nm) and B (489 nm) as shown in FIG. 36. The wavelengths of the R and B components are set so that the luminosity of the R and B components is 20% of that of the G component.
As shown in FIG. 35, the G and B components of the first-order light have particularly weak spectra, producing subjectively unnatural blurring in defocusing. The apex of the projection in each hexagon is theoretically acute but in practice such a shape is difficult to attain by presently available machining techniques. Therefore, the variations in spectral intensity tend to be greater than theoretical values.
FIG. 37 shows another example of conventional focusing screens, which is also provided with a pattern of regular hexagons with the inter-apex distance of 17.2 .mu.m and which has a generally spherical projection formed in each hexagon (the calculation of spectra is approximated by a stepped pattern as shown in FIG. 37). The characteristics of this focusing screen are as shown in FIG. 38. As in the previous case, the balance between the zero-order and first-order light is poor for all color components, producing unnatural blurring in defocusing.
Assume in this case that a reflex type telephotographic lens (mirror lens) having a large F number is attached to a single-lens reflex camera. As shown in FIG. 39, the exit pupil (E.P.) of the mirror lens is of annular shape, so the first-order light L.sub.1 falls on the focusing screen (F.S.) but the zero-order light L.sub.0 will not. Therefore, if the focusing screen used has such poor color balance that the balance of spectral intensity between the zero-order and first-order light is reversed with respect to the R and B components, an uneven color distribution will occur in the central portion of the finder when looked at by the viewer's eye E through an eyepiece lens E.L.
The technique disclosed in Unexamined Published Japanese Patent Application No. 41728/1982 is known as a method for forming a microstructure array of a type that can be employed in the focusing screens described above. The exposing process employed with the technique described in this patent application comprises the following steps. Placing a transmission chart on which a pattern is drawn, the elements of which are equal in number to those of the fine pattern to be formed on the layer of light-sensitive material, between a light source and a layer of light-sensitive material. Forming an image of the chart on the layer of light-sensitive material via an imaging lens, thereby producing a pattern of fine asperities in accordance with the intensity of light.
In this process, a pattern which contains as many elements as the fine pattern of asperities to be formed, must be formed on the transmission chart, and this inevitably adds to the complexity of the chart fabrication process. Furthermore, if one wants to modify the geometry of the pattern of asperities, he has to remake the whole pattern to be formed on the chart.
In the event that the distance between the layer of light-sensitive material and the transmission chart being placed between it and a light source varies, then variations would also result in the geometry of the pattern of asperities to be formed. Therefore, close tolerances are required when placing the chart between the light-sensitive layer and the light source.
A further problem occurs when a very complex pattern of asperities is to be formed on the layer of light-sensitive material, since in order to form a pattern of high and low spots which faithfully reflects the transmission characteristics of the pattern formed on the transmission chart, the distance between the light-sensitive layer and the chart must be set to a considerably small value during exposure. However, if this distance is reduced, interference fringes will occur between the light-sensitive layer and the chart, thereby making it impossible to obtain the desired regular pattern of asperities. If the resulting pattern is used with a focusing screen, partial unevennes in diffusion will inevitably occur.